The present invention relates to oscillators generally and, more particularly, to a programmable oscillator configured to generate a stable frequency reference on a chip without using external crystals or resonators.
One conventional approach used to implement an oscillator without a crystal is to use a simple resistor/capacitor (RC) network to implement a timer. The original 555 timer chip design used an RC network. However, RC networks are susceptible to process variations and temperature variations. A typical mainline CMOS process does not control resistors or capacitors to tolerances of better than 5%. In some processes, the tolerance is even lower. Laser trimming and other techniques can be used to achieve higher tolerances, but may add to the overall cost of the device. An example of a modification of such a circuit can be found in U.S. Pat. Nos. 5,565,819 and 5,670,915, which are hereby incorporated by reference.
A second conventional approach used to implement temperature insensitive current sources is described by R. A. Blauschild in his paper entitled AN INTEGRATED TIME REFERENCE, Proceedings of the IEEE Solid-State Circuits Conference, Feburary 1994, pp. 56-57, which is hereby incorporated by reference. Such an approach develops a temperature invariant current by using a bias generator that sums currents with different temperature coefficients and combines them with a threshold cancellation circuit. The technique allowed a current that was proportional to oxide thickness. This method was applied to time interval measurement and to filtering, but not to oscillator design.
A third conventional approach used to implement an oscillator is to use a ring oscillator that is stable across process and temperature variations. This is often used in timing recovery PLL circuits. The ring oscillator approach appears to be able to achieve frequency stability on the order of 5%, which is not good enough for a target of 2% or less.
Referring to FIG. 1, a portion of a ring oscillator 10 is shown. The ring oscillator 10 comprises a number of devices 12a-12n. FIG. 2 generally illustrates the temperature dependence of the frequency of oscillation of the devices of the ring oscillator 10. The temperature dependence of the ring oscillator 10 adversely affects the frequency of oscillation.
Referring to FIG. 3, a circuit 20 is shown illustrating a biasing circuit for a delay cell that may be used with a conventional ring oscillator. A delay cell 22 generally presents a signal VDD, a signal PBIAS, a signal BIASA, a signal BIASB and a signal VSS to a biasing circuit 24. The biasing circuit 24 may include a current source 26 that responds to the signal PBIAS. The biasing circuit 24 may provide biasing to a voltage reference circuit 28 that is used as a VCO input. Additionally, a bandgap current bias circuit 30 provides additional biasing to the voltage reference 28. However, while the circuit 20 may be roughly temperature independent, it does not generally provide a high precision frequency of oscillation (i.e., less than 2%).
The present invention concerns an oscillator circuit configured to generate an output signal having a frequency comprising a current source, a trim circuit, and one or more capacitors. The current source may be configured to generate a temperature independent current in response to a first adjustment signal. The trim circuit may be configured to generate the first adjustment signal. The one or more capacitors may be configured to charge to a controlled voltage using the temperature independent current. The controlled voltage may regulate a variation of the frequency of the output signal.
The objects, features and advantages of the present invention include providing a circuit and method that may implement a precision on-chip current controlled oscillator. The present invention provides an accurate programmable oscillator that (i) provides accurate frequencies (e.g., in the order of 2% or less), (ii) eliminates the need for a resonator or a crystal oscillator, (iii) may be used with a microcontroller to provide a single-chip clocking solution for an entire system.